Research topic 2

2) Electronics and optoelectronics on 2D van der Waals materials

High-Resolution Observation of Nucleation and Growth Behavior of Nanomaterials Using a Graphene Template

By using graphene as an electron beam-transparent substrate for both nanomaterial growth and transmission electron microscopy (TEM) measurements, we investigate initial growth behavior of nanomaterials. The direct growth and imaging method using graphene facilitate atomic-resolution imaging of nanomaterials at the very early stage of growth. This enables the observation of the transition in crystal structure of ZnO nuclei and the formation of various defects during nanomaterial

Epitaxial GaN Microdisk Lasers Grown on Graphene Microdots

Direct epitaxial growth of inorganic compound semiconductors on lattice-matched single-crystal substrates has provided an important way to fabricate light sources for various applications including lighting, displays and optical communications. Nevertheless, unconventional substrates such as silicon, amorphous glass, plastics, and metals must be used for emerging optoelectronic applications, such as high-speed photonic circuitry and flexible displays. However, high-quality film growth requires good matching of lattice constants and thermal expansion coefficients between the film and the supporting substrate. This restricts monolithic fabrication of optoelectronic devices on unconventional substrates. Here, we describe methods to grow high-quality gallium nitride (GaN) microdisks on amorphous silicon oxide layers formed on silicon using micropatterned graphene films as a nucleation layer. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots with intermediate ZnOnanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaNmicrodisk with a Q-factor of 1200 was observed at room temperature.

Position- and Morphology-Controlled ZnO Nanostructures Grown on Graphene Layers

Position- and morphology-controlled ZnO nanostructures are grown on an oxygen plasma-treated selective area of graphene layers using metal-organic vapor-phase epitaxy. The structural and optical characteristics examined by electron microscopy, cathodoluminescence and photoluminescence techniques indicate that high-quality nanostructures are prepared on graphene layers. This approach to grow the controlled ZnO nanostructures selectively on graphene layers enables us to fabricate variousnanodevices including GaN/ZnO coaxial nanotube LED microarrays.

High-quality GaN films grown on chemical vapor-deposited graphene films

We report the growth of high-quality GaN films on large-size graphene films for visible light-emitting diodes (LEDs). The graphene films were synthesized by chemical vapor deposition and then transferred onto amorphous silica (SiO2) substrates that do not have an epitaxial relationship with GaN. Before growing the high-quality GaN thin films, ZnO nanowalls were grown on the graphene films as an intermediate layer. The structural and optical characteristics of the GaN films were investigated, and the films exhibited stimulated emission even at room temperature, a highly c-axis-oriented crystal structure, and a preferred in-plane orientation. Visible LEDs that emitted strong electroluminescence under room illumination were fabricated using the GaN thin films.

Flexible Inorganic Nanostructure Light-Emitting Diodes Fabricated on Graphene Films

Flexible inorganic nanostructure light-emitting diodes (LEDs) are fabricated using high-quality GaN/ZnOcoaxial nanorod heterostructures grown directly on large graphene films. The nanostructure LEDs fabricated on graphene films are readily transferred onto flexible plastic substrates, which operated reliably in a flexible form without significant degradation of the LED performance.

Microstructures of GaN Thin Films Grown on Graphene Layers

Plan-view and cross-sectional transmission electron microscopy images show the microstructural properties of GaN thin films grown on graphene layers, including dislocation types and density, crystalline orientation and grain boundaries. The roles of ZnO nanowalls and GaN intermediate layers in the heteroepitaxial growth of GaN on graphene, revealed by cross-sectional transmission electron microscopy, are also discussed.

Transferable GaN Layers Grown on ZnO-Coated Graphene Layers for Optoelectronic Devices

We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) usinggraphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films ongraphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphenesubstrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.